Embodiments of the inventive concepts described herein relate to a method for manufacturing anodic metal-oxide nanoporous templates, and more particularly, relate to a method for manufacturing anodic metal-oxide nanoporous templates through a highly efficient and eco-friendly process.
When an electric field is applied to a metal in an acidic electrolyte, a nanoporous anodic oxide layer is formed on the surface of the metal. Such phenomena are defined as anodization.
FIG. 1 illustrates a nanoporous anodic oxide layer formed on the surface of a metal.
A nanoporous anodic oxide layer has a honeycomb structure in which hexagonal unit cells are periodically arranged as shown in FIG. 1. At the centers of the unit cells, nanopores are present with relatively large aspect rations.
Anodization is traditional technology of forming a protection layer for preventing a metallic surface from corrosion. In recent years, many studies are sprightly progressing for applications to biotechnology, energy storage, filters, and nanoporous templates for fabricating functional nanostructures.
For these applications, an anodic metal oxide, in which nanopores are uniformly arranged over a large area, should be needed. And addition procedures, such as detaching a fabricated anodic oxide layer from a metal substrate and removing a barrier oxide layer to open both sides of the nanopores, would be required.
The 2-step anodization reported by H. Masuda et al. is that a mild anodizing process is repeated twice, resulting in a nanoporous anodic aluminum oxide (AAO) layer with superior periodicity over a relatively large area.
For example of fabrication, after texturing an aluminum surface by removing an AAO, which is formed by a pre-anodizing process, through a main etching, a main anodizing process may be further executed to periodically concentrate an electric field by anodic bias.
As a result, nanopores with a uniform diameter are formed in centers of hexagonal unit cells. An electro-polishing process for reducing surface roughness of aluminum contributes to shortening a time for texturing. The most general method of detaching such an AAO from a remaining aluminum is to dissolve the remaining aluminum in a solution of mercury chloride (HgCl2) or copper chloride. Before chemical dissolution of the remaining aluminum substrate, a process of coating an upper part of the AAO (the opposite side of a barrier oxide) with an organic material might be needed to prevent an aluminum-removing reagent from infiltrating into nanopores. Then, a process of removing an oxide layer barrier or widening nanopores is optionally performed to adjust a detached AAO for the application.
The conventional technology consisting of AAO fabricating and separating procedure has a couple of drawbacks, which are time-consuming procedure, utilizing a reagent poisonous to human bodies and environments, and inefficient usage of resources.
In a view point of a fabricating time, conventional technology generally adopts mild anodizing scheme exhibiting relatively slow AAO growth rate, which has to repeat twice in 2-step anodization method. In addition, dissolving time for separating AAO should be considered, which is proportional to a thickness of remaining aluminum.
A hard anodizing (HA) process, proposed to overcome such a problem, is useful to greatly improve the growth rate and uniformity of an AAO, but it is necessary to prepare an expensive cooling device for dissipating heat generation due to a high anodic current. Furthermore, nanopore diameter in AAO fabricated from HA process is relatively small comparing with that from MA, which is restrictive to its potential applications.
Moreover, HgCl2 used in the AAO separation is highly toxic to human bodies and environments. In the case of using a thin specimen for reducing a dissolving time determined by a thickness of aluminum, it is difficult to handle the specimen during whole procedure.
Although recent reports about directly detaching an AAO from aluminum using pulse-type anodic bias, these technologies are inevitable to use highly reactive and dangerous reagents such as butanedione or perchloric acid based detaching electrolytes. Additionally, because an anodic electrolyte is different from a detaching electrolyte, more solid washing/cleaning step should be added thereto to increase the complexity of process.
Furthermore, the conventional AAO detaching technology could not reuse the metal (e.g., aluminum) specimen because remaining part is wasted by dissolving it away.
Finally, the aforementioned conventional technologies can only produce one AAO through the full process because they are just applicable to a mono-surface of an aluminum specimen. And, in the case of using a polygonal specimen, it is necessary to apply a process or specimen holder for preventing other surfaces but a target surface from anodization.